1. Field of the Invention (Technical Field)
The present invention relates to the precise measurement of very small rotational motions without being sensitive to translational motions, as, for example, in measuring very small forces using a rotational torsion balance in a noisy vibrational environment.
2. Background Art
Known prior art interferometer-based measurement techniques lack the ability to accurately measure small distances in vibrationaly noisy environments. The first is the standard interferometer. The standard interferometer employs one mirror, which is stationary, while another mirror is attached to a torsion balance bar. That device is highly sensitive to vibrations because the fringes move when the moving mirror approaches or recedes from the laser.
Another known interferometer-based measurement technique is the etalon which has a thick plate of transparent material with two parallel surfaces. That device has the disadvantage that the second ray or beam involved in the interference pattern results from only one direct round trip through the plate rather than two, thus reducing the accuracy of measurements obtained therewith. In order to provide an etalon which has a thickness sufficient to allow a round trip which has a distance comparable to that obtained with a corner prism, the etalon plate must be about three times as thick as the depth of the prism. An etalon having such a thickness is thus large and bulky. Further, a reflective optical coating is required on the etalon's second face in order for effective operation to be provided. Such an optical coating further increases the cost of an etalon-based system.
Finally, an etalon which is created from two parallel plates with reflective and antireflective coatings separated by an air or vacuum space can be used for interferometer-based measurement techniques. In this etalon, as with that described above, the second ray or beam results after only one direct round trip through the device. Furthermore, since the path is through air or vacuum rather than glass, the plate separation must be not just about three but as much as five times greater than the depth of the corner cube prism to achieve the same sensitivity, thus adding substantially to the bulk of the device. Also, an additional structure is required to keep the plates parallel. Finally, the corner cube prism requires no optical coatings, in contrast to both of the etalon configurations, thereby substantially reducing the cost of the final interferometer.
Previously, building and using a torsion balance to measure very small forces in a microthruster development program were reported by the inventor. See C. R. Phipps, et al., “Laser ablation of organic coatings as a basis for micropropulsion”, Thin Solid Films, 453-4, 573-83 (2004); and C. R. Phipps, et al., “Diode Laser-driven Microthrusters: A New Departure for Micropropulsion”, AIAA Journal, 40, no. 2, pp. 310-318 (2002). A torsion balance is a string which suspends a balanced, horizontal bar with an object which generates a small force mounted on one end of the bar. The force applied to the end of the suspended horizontal bar causes the bar to rotate. The magnitude of rotation experienced by the bar is directly proportional to the force exerted on the end of the bar. Previously, the rotational motion due to the force was determined by noting the position on a fixed chart of the reflection of a 1 mm diameter laser beam from a small mirror mounted on the center of the bar. However, that technique precludes the ability to obtain measurements of less than about 10 micronewtons without exceeding about 10% accuracy. The limitation on the ability to obtain accurate measurements with that technique is due primarily to the diffraction of the laser beam. There is thus a present need for a method and apparatus for accurately measuring small forces.